General relativity passes a large scale test

A new look at general relativity finds that it passes with flying colors at …

General relativity, our current best understanding of gravity, has passed yet another test—this time on a much larger length scale. Ever since relativity's first confirmation in 1919, when Sir Arthur Stanley Eddington observed that the light from distant stars was shifted by the mass of the sun, direct tests have been confined to length scales smaller than our solar system. No test to date has stringently probed general relativity's applicability to the length scales of the universe itself.

A paper that is published in the current edition of Nature reports on research that incorporates gravitational lensing, galaxy clustering measurements, and growth rates of large scale structures to measure a single parameter that can be compared to the predictions of general relativity. To probe the effect of gravity at large length scales—on the order of two to 50 megaparsecs (MPc) at a redshift of 0.32—the authors describe a variable EG that incorporates three physical parameters and can be used to differentiate between competing theories of gravity.

The bulk of the matter in the universe (dark matter) cannot be seen, which would seem to cause problems for any direct measurement that is using electromagnetic radiation as its input. But EG is insensitive to "galaxy bias" and matter perturbations on the length scales of interest. This means that, in their method, no second measurement is needed to estimate how lumpy a given region of space is. The "source" of the gravity, be it dark or normal matter, is irrelevant, making the method more robust and less dependent on models than other proposed tests.

The authors used data from the Sloan Digital Sky Survey that encompasses 70,205 distinct galaxies, covering over 5200 square degrees of the night sky. The galaxies represent a redshift range of 0.16 at the recent end to 0.47 at the far past end, corresponding to a median look back in time of 3.5 billion years, when the universe was about 77 percent of its current size. Using these survey galaxies as inputs, the authors arrive at an EG parameter equal to 0.39 ± 0.06.

Calculating the value using general relativity and the ΛCDM model of our universe produces a value of 0.408 ± 0.029. The calculation and measurement statistically overlap each other at one standard deviation. This suggests that general relativity holds true, even at length scales 1011 times greater than previous tests.

However, general relativity is not alone in the world of cosmological theories. Any of the Modified Newtonian Dynamics (MOND) gravity theories, or other extensions to general relativity, must also be capable of generating an EG value in the range of the measured one to be considered valid.

Testing a flavor of a version of MOND known as tensor-vector-scalar (TeVeS) theory, the team computed an EG value of 0.22, which is lower than the measured value by more than 2.5 standard deviations. Similar calculations using f(R) theory—an extension of general relativity—reveal an EG value of between 0.328 and 0.365. Not exact, but not far enough outside the error bars to be ruled out as statistically unlikely.

As is often the case, more data from future sky surveys will help to reduce the uncertainty in the measured value. But even the current value places another hurdle in the way of any theory of the universe that seeks to challenge general relativity. Not only must any new theory be capable of explaining what general relativity does, but it must be able to reproduce this new constraint as well.

Matt Ford / Matt is a contributing writer at Ars Technica, focusing on physics, astronomy, chemistry, mathematics, and engineering. When he's not writing, he works on realtime models of large-scale engineering systems.